Thanks for adding multi leagues, its going to be allot of fun seeing who the best player is in each league. As for the Collection Score, total DEC value likely isn't the best way to go as it feels as if its another step in the pay to play direction.
My suggestion for the issue at had is Summoner/Monster levels. For example .... anyone who wishes to battle in the Diamond league would need to have at least one maxed summoner and X maxed monsters. This would stop any level one and other weaker teams from reaching Diamond +. The same concept can be applied to all the league levels. This suggestion would also allow those with a mixed bag of leveled cards and Skill to still compete at the highest of levels.